The present invention relates in general to the field of scanning mirror interferometers, and in particular to a system for correcting vibration-induced errors in the movement of the scanning mirror.
Fourier Transform Infrared (FTIR) Spectrometers have achieved a widespread degree of popularity for the spectral analysis of chemical compositions. At the heart of the FTIR spectrometer is a Michelson Interferometer in which light from a broadband infrared source is split into two beams, to be reflected off a fixed mirror and a moving mirror respectively. The beams are recombined and irradiate an unknown sample, before impinging on a detector. The intensity of the recombined beam as a function of moving mirror position, which is made up of contributions of different wavelengths within the infrared source, is known as an interferogram. Performing a Fourier transform on the interferogram yields a spectrum identifying the infrared-absorbing constituents of the sample.
The accuracy of a continuous scan interferometer spectrometer is closely related to its ability to translate the moving mirror at a constant speed, and in a direction such that its reflecting surface remains normal to the light beam incident thereon. When used in a laboratory environment the instrument generally can be isolated effectively from external vibrations which otherwise would introduce fluctuations in the mirror speed and angular position. However, FTIR spectrometers are being used increasingly as airborne instrumentation, for example in space research, and also in on-line situations where they are coupled to a manufacturing process to insure consistent product quality. In these latter two situations, external vibrations are quite common, and so compensating schemes must be included within the spectrometer to negate the effects of the vibrations.
Several techniques have been used in the prior art to provide vibration compensation. One approach has been to sense the deviation in the tilt or the velocity of the moving mirror from predetermined settings by comparison with reference signals, and to compensate for the deviations, using either mechanical or mathematical techniques. In the mechanical compensation mode, error signals derived from the instantaneous moving mirror position are fed back through a servo system to one or more of a variety of mechanisms which readjusts the mirror tilt or velocity. In some cases, the compensation is applied directly to the main drive mechanism of the moving mirror, as shown in U.S. Pat. Nos. 3,488,123 and 4,149,118. In other cases auxiliary mechanisms, coupled to either the moving or the fixed mirror apply the necessary corrections, as disclosed in U.S. Pat. No. 3,809,481 and in "Fourier Spectroscopy Applied to Field Measurements," by G. W. Ashley and A. G. Tescher. Special Reports No. 114, Aspen International Conference on Fourier Spectroscopy, Jan. 5, 1971.
In the mathematical mode, electronic, or similar, circuitry subtracts the effects of mirror tilt or velocity deviations from the spectral information as it is being processed.
Another common approach is to use a rigid, precisely machined transport mechanism for the moving mirror which, due to its rigidity, does not permit the mirror to deviate significantly from its desired path, or from its desired speed. This approach may entail the use of lead screw or air bearing mechanisms coupled with a mirror-bearing carriage which rides along a precisely machined track, the close tolerances between the carriage and the track substantially eliminating fluctuations in mirror position. Nevertheless, an auxiliary servo mechanism, coupled with a feedback loop, usually is needed to adjust for mirror deviations. Examples of such hybrid systems are shown in U.S. Pat. No. 4,053,231 and in "Interferometer Design and Data Handling in a High-vibration Environment--Part I Interferometer Design" by R. P. Walker and J. D. Rex, SPIE Vol. 191 Multiplex and/or High-Throughput Spectroscopy (1979).
There have been deficiencies in these prior art attempts to solve the vibration problem. The precisely machined mirror transport mechanisms are generally quite expensive to manufacture, and are susceptible to wear, which erodes the accuracy of the interferometer. Therefore, continual attention and maintenance are required. On the other hand, the feedback schemes which correct for deviations from desired mirror tilt and velocity are often quite complex and expensive, and are limited as to the range of vibration frequencies to which they can respond. The deviations in mirror movement are usually sensed by observing fluctuations in the periodicity of the zero crossings of the interference fringes. However, since there is not a continuous observation of mirror position, but only at the two times per cycle when the interference fringes experience a zero crossing, the responses of such feedback schemes have been severely bandwidth-limited.
Also, a particular compensating mechanism typically corrects for either velocity or tilt, but not for both. Therefore multiple schemes are needed to adjust both parameters simultaneously.
Therefore, in light of the foregoing it is an object of the present invention to provide a simplified vibration-compensating mirror drive system which simultaneously controls both mirror tilt and velocity, and does so in a highly reliable, easily maintainable and relatively inexpensive fashion.
It is a further object of the invention to achieve improved bandwidth in a vibration-compensation scheme by a continuous derivation of mirror position and velocity data, so as to compensate for vibrations having a wide range of frequencies.